1. Field of the Invention
This invention relates to series resonant converters and more particularly to such a converter which includes therein a parallel resonant circuit to limit the converter's operating frequency.
2. Description of the Prior Art
Power converters are known in the art and typically serve to accept energy from an unregulated source, such as a voltage source, and derive therefrom a regulated voltage which is applied to a load. The regulation function is performed by interposing a regulating device between the source of energy and the load. One such regulating device known in the art is a switching type device. These operate in a discontinuous manner in controlling the rate of energy transmission. The switching device has two modes of operation, fully on and fully off. The switching device is periodically turned on for a time interval to permit energy transfer for purposes of maintaining the power output at a predetermined level.
Typically, such switching type regulating devices employed in power converters utilize semiconductor devices, such as power field effect transistors (FETS), as the switching devices. These devices are turned fully on, or saturated, or fully off during operation. When fully on, the semiconductor devices are conducting and little or no power is dissipated. Also, when nonconducting or fully off no power is dissipated therein. Power is, however, dissipated in such a semiconductor device during the time interval of switching from a nonconducting condition to a conducting condition and vice versa. It is during the switching time interval that a substantial amount of power may be dissipated in such a semiconductor device, and if large enough this may severely damage the semiconductor device.
Power converters employing switching type regulators utilizing semiconductor devices as power switches have typically been operated such that the power switches supply a squarewave signal to a power transformer which couples the power converter to a load. The squarewave operation requires that the power switches dissipate energy whenever voltage and current are interrupted. Consequently, there is a switching loss which is directly related to the operating frequency of the power switches. This has contributed to limiting such power switches to low power applications during high frequency operation. Otherwise, they are operated at a low frequency using larger components and more space.
It is desirable to provide power converters which are cost effective and which occupy a small amount of space. This, then, necessitates high frequency operation, such as in excess of 20 KHz and preferably at substantially higher frequencies, such as 200 KHz, while transferring large amounts of power, such as on the order of 1500 watts or more. In order to achieve such high frequency operation of power switches, it has been proposed to employ sinusoidal operation as opposed to the squarewave operation typically employed in the prior art. Such a proposal is found in the article "High Frequency Power Conversion With FET-Controlled Resonant Charge Transfer", by R. H. Baker, published in the PCI April, 1983 proceedings, pages 130-133. This article proposed an operation wherein sinusoidal current pulses flow through a series resonant circuit including the primary winding of a power transformer by way of a power switch. Each sinusoidal current pulse terminates to a zero level before the voltage forcing function is withdrawn. As a consequence, the power switches turn on and off at zero current to thereby provide lower component switching loss to reduce component stress.
As reported in Baker, supra, alternate sinusoidal current pulses flow through a series resonant tank circuit. During one half cycle a current pulse flows in a first direction through the primary winding by way of a first power switch and a first capacitor. During the next half cycle a current pulse flows in the opposite direction through the transformer primary winding by way of a second power switch and a second capacitor. These two power switches are each turned on for a fixed time duration with the on times being separated by a minimum fixed interval or dead time during which one switch is turned off and the other is turned on.
The power switches in Baker, supra, are turned on and off at a frequency which varies from a low frequency, for low loading or low power operations, to a high frequency, for high loading or high power operations. At high power operations such a converter operates near its maximum resonant frequency. If the power requirements drop, the operating frequency will drop. Depending upon the application for which such a converter is employed, this variable operating frequency may present problems. If the converter is employed as a high power converter, such as 1500 watts at an operating frequency of 200 KHz, a drop in the loading may result in an objectional operating frequency. For example, if such a converter supplies power to telephone lines, a drop in loading may cause the operating frequency to fall within the telephone voice band, namely, within a frequency range on the order of 300 Hz to 3400 Hz. The ripple voltage supplied to the telephone lines may, during this operating frequency range, inject objectional audible noise onto the telephone lines.
One solution to the problem of audible noise is set forth in U.S. Pat. No. 4,587,604 entitled "A Power Supply Employing Low Power And High Power Series Resonant Converters". As set forth therein, the supply includes both high and low power series resonant converters. Each of the converters include a pair of power switches which are alternately turned on and off. The lower power converter operates in parallel with the higher power converter during high power operations. For low power operations a control operates to disable the higher power converter so that only the low power converter supplies power to the load. While this combination of low and high power converters has been found to operate so as to prevent the resonant frequency from dropping into the audible range, it does require that the supply include what is in effect an extra set of power switches as well as associated control circuitry. Therefore, it is desirable to prevent the resonant frequency from dropping into the audible range without the necessity to add the extra pair of switches and associated circuitry and also without affecting the operation of the FETS used as the power switches in the resonant converter.